The present invention relates to a spray coating device for spraying pulverulent to granular consistency particulate coating material and particularly to means for removing from the coating chamber the coating material which accumulates on the walls of the chamber.
The spray coating device includes a coating chamber to receive the articles to be coated. An article spraying device is directed into the interior of the coating chamber. The chamber has a discharge device for excess coating material which accumulates on the floor of the chamber comprising at least one driven endless belt to take up accumulated coating material.
During spray coating of articles, a portion of the coating material is sprayed past the article or bounces off it. This material need not be lost. Instead, it can be recovered by means of suitable filtering and treating devices. Upon a change in the coating material, particularly a change from one color to another color, all traces of the previous coating material must be removed before spraying begins with a new material. Otherwise, coating defects, and particularly color defects, may result. Cleaning during the change of the coating material results in undesired stopping or down times. When the coating is being done with powder, as little powder as possible should be present at any time in the chamber since large accumulations of powder may, in case of subsequent eddying, form an ignitable air-powder mixture.
Various attempts at solving these difficulties have already been made.
(a) A movable belt may form the floor of the coating chamber. U.S. Pat. No. 3,252,401 discloses a moving endless belt, which forms the floor of the coating chamber, and describes the floors as receiving the falling excess powder. The endless belt moves between rolls and unwinds from one roll and is wound up on another roll. Thereafter, the belt is either thrown away or is cleaned for reuse. An endless rotating filter belt used as the floor of the chamber is shown in German Pat. No. 2,430,517, which corresponds to U.S. Pat. No. 3,902,455. In the coating chamber here, a special cleaning device is provided which draws all powder particles of a given color from the filter floor. Subjecting such a filter belt to suction both below and above the powder-covered section of the belt is known from German Auslegeschrift No. 2,546,920, which corresponds to U.S. Pat. No. 3,918,641. The use of an endless impervious conveyor belt for removing powder from the chamber is known from German Offenlegungsschrift No. 2,153,395. All of these systems, however, have the disadvantage that the walls and roof of the coating chamber must still be cleaned by hand.
(b) With relatively open coating chambers, a wall is placed behind the articles to be coated. The wall has the form of a moving belt that intercepts the powder particles which are sprayed past the article. In U.S. Pat. No. 2,232,561, the belt is an endless complicated link chain, the lower end of which travels through a cleaning bath. In U.S. Pat. No. 3,744,450, there is an endless rotating belt which is arranged as the powder collection wall behind the article to be coated. This belt must be replaced upon each color change from a powder of one color to a different powder of a different color. In U.S. Pat. Nos. 2,841,073 and 2,875,680, the wall is a downwardly traveling endless belt, which must be replaced when it has run off from the feed roll.
(c) If stationary coating chamber walls are provided with air passage openings, as proposed in Swiss Pat. Nos. 560,558 and 581,503, the chamber walls will remain clean. However, this has the disadvantage that the scavenging air which enters through the walls combines with the powder-air mixture in the coating chamber and thus reduces the coating effectiveness of the chamber. The incoming scavenging air dilutes the cloud of coating powder. Furthermore, the scavenging air results in eddying and depositing of powder in "dead" corners. Finally, a large amount of scavenging air and thus also a high consumption of energy are necessary.
(d) One solution calls for cleaning the chamber walls. Although this is technically simple, it is cumbersome in practice. As shown in German Gebrauchsmuster No. 78 22 478, the process comprises hanging replaceable walls which are changed upon a change in color or powder in the coating chamber.
(e) German Offlegungsschrift No. 2,132,946 shows an endless, air-pervious belt, which extends over the length of the coating chamber and travels endlessly in the chamber transverse to the chamber over the floor, the two side walls and the roof. Below this endless belt, there is a conveyor belt for removing excess powder from the chamber. The endless belt which forms the inner chamber walls does not transport any powder out of the chamber but serves to produce an eddying of powder within the chamber.
(f) U.S. Pat. No. 2,509,276 shows a coating chamber through which a web of articles to be coated passes between an electrically grounded chamber roof and endlessly rotating, driven, high-voltage electrodes. The high-voltage electrodes are located within a U-shaped loop, which is open toward the top of an endless rotating grounded grid electrode. The coating of the bottom side of the web of material is effected by a spray device which is arranged between the high voltage electrodes and the grid electrode on the one hand and the web of goods on the other hand. The electric field of the high-voltage electrodes causes the coating material to be attracted by the grounded parts. Therefore, the greater part of the coated material is attracted by the web of goods, as a result of the grounded roof of the chamber, and the excess by the grid electrode. From this, the coating material adhering to it is removed at the lower end of the chamber by brushes and scrapers.